1. Field of the Invention
The invention involves improvements in ion implantation techniques. It is directed to systems for controlling the ion dosage of work pieces such as semiconductor wafers. These systems utilize a rotary disk which carries the wafers to be irradiated around its periphery and rotates them past an ion beam. As the wafers are irradiated, a second mechanical displacement of the disk occurs in a radial direction so that the entire surface of each wafer is subjected to the beam and receives the proper dosage.
2. Description of the Prior Art
A known technique for implanting ions in wafers utilizes a rotating disk mounted in a vacuum chamber and adapted to carry a set of wafers around the periphery of the disk such that by rotating the disk a portion of each wafer is successively exposed to an ion beam which is projected into the chamber. A second drive system moves the rotating disk assembly in a radial direction so that successive adjacent tracks of each wafer are subjected to the ion beam until the entire surface of each wafer has been implanted.
Patents illustrating this technique include U.S. Pat. No. 4,234,797 issued to Jeffrey Ryding and U.S. Pat. No. 3,778,626 issued to Gordon Ian Robertson.
In one mode of the control system of the Robertson patent, the ion beam is monitored by measuring the current which flows to the disk itself by reason of its interception of the beam. This current is measured through a slip ring system and is supplied to a control system which also senses the radial displacement of the disk relative to the beam. Also supplied as an input to the control system is a desired dosage signal. With these inputs the Robertson control system controls the radial motion of the disk to control the ion distribution over the wafer surface.
The Ryding patent employs a current measurement arrangement which uses, instead of slip rings, a Faraday cage mounted behind the disk and located to receive pulses of the beam current when an aperture in the disk passes in front of the cage exposing it to the ion beam. Ryding employs the measured beam current to control the radial velocity of the rotating disk for the purposes of achieving a uniform ion dosage. While Ryding's technique eliminates the need to sense radial displacement, his process depends upon maintaining angular velocity at a constant value thus introducing a possible source of error.